Electrical converters, such as active rectifiers and inverters, are used for converting a first current into a second current of different frequency, for example for supplying a load, connecting a power source to a grid or for interconnecting two electrical grids.
Specific medium voltage converters are linked with a low pass input or output filter that is passively damped through a large resistor. Typically, the filter comprises a filter inductor and a filter capacitor with a damping resistor in series. Such a filter is known as LCR filter. However, such a structure may entail substantial power losses through the resistor and may not provide an adequate current profile at the output due to slow decay rate in attenuation capabilities at higher frequencies.
Thus, a filter without a resistor (called LC filter) is very attractive from an input-output point of view, since the converter is required to have certain machine friendly behavior, exhibited through low current Total Harmonic Distortion (THD), as well as grid code compliance, exhibited through low current and/or voltage THD and individual harmonic constraints satisfaction.
From the control point of view, with only an LC filter it may be difficult to ensure stability of the closed-loop system (converter, attached cable, transformer, electrical machine, etc.). Due to the presence of a resonant peak of the LC filter, higher order resonances of the system may be amplified and the system may develop oscillations.
Possible control methods for controlling the converter may be model predictive control methods. For example, EP2469692A1 describes model predictive pulse pattern control (MP3C), which, for example, may provide fast closed-loop control of an AC machine with an N-level voltage source inverter. MP3C relies on optimized pulse patterns (OPPs) with low total harmonic distortion factors that are computed online. The OPPs are used to generate reference flux trajectories that are to be followed. MP3C comprises an online computational stage that adjusts the switching instants in the OPPs so as to maintain the flux on the reference trajectory in closed-loop.
U.S. Pat. No. 5,734,249 discloses a generic method for controlling an electrical converter for an electrical drive.
Furthermore, in “State of the Art of Finite Control Set Model Predictive Control in Power Electronics”, IEEE Transactions on Industrial Informatics, 2012 discloses the possibility of using a cost function for controlling a power system with a voltage source inverter with a passive output filter, such as to damp resonant oscillations of the filter.